The traveling wave tube is a type of microwave device which is widely used as a component of microwave electronic systems to both amplify and generate microwave frequency electromagnetic waves. In the traveling wave tube, a stream of electrons is directed along a slow wave structure of the device. A microwave frequency electromagnetic wave is made to propagate along the slow wave structure. This structure provides a path of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure so that the traveling wave may be made to propagate axially at nearly the velocity of the electron beam. The interaction between the electron beam and the electromagnetic wave causes velocity modulations and bunchings of the electrons in the beam. Energy is thereby transferred from the electron beam to the electromagnetic wave traveling along the slow wave structure, thereby amplifying the electromagnetic wave.
One widely used type of slow wave structure used in traveling wave tubes is the coupled cavity structure. In this type of slow wave structure a series of interconnected cavities are aligned along a common axis. The electron beam passes along the axis through apertures in the cavities. One of the problems encountered in traveling wave tubes of the coupled cavity variety is the high cost of construction resulting from the fact that a coupled cavity tube uses many identical cavities each of which must be manufactured to a vary high precision. Another problem encountered in such tubes is that the well known impedence-bandwidth product (or equivalently, the gain per wavelength-bandwidth product) is somewhat reduced because the cavities tend to shield the electron beam from the fields over portions of the beam length thereby reducing the effective interaction between the beam and the traveling wave.
Another widely used type of slow wave structure is a helix which is usually supported within a tubular housing by means of a plurality of axially spaced dielectric rods equally circumferentially spaced about the helix. One of the desirable operating characteristics of such a slow wave structure is that it is capable of amplifying input microwave signals over an extremely broad bandwidth, typically an octave or more. However this very advantage of such a slow wave structure sometimes becomes one of its weaknesses because the bandwidth-impedence relationship is essentially a constant. Therefore, the dielectrically supported helix, having a large bandwidth, must necessarily have a low effective interaction impedence.
The smaller the interaction impedence, the smaller the radius of the slow wave structure must be in order that the wave to be amplified is in close proximity to the electron beam with which it is interacting. For example, a helix for operation at one-half centimeter wavelength has a diameter of several thousandths of an inch. Such a structure is too small to be practically constructed even if it were its power dissipation would be so small as to be virtually useless. Thus, although the dielectrically supported helix is a very practical circuit when amplifying waves longer than, for example, one or two centimeters in wavelength, at shorter wavelengths the resulting helix would be so small as to be highly difficult if not impossible to construct.
Another serious deficiency of the dielectrically supported helix is that it has a limited power output capability because the dielectric support rods can conduct away from the helix only limited amounts of heat generated by beams interception and r-f losses before damage results.
Ring-plane and helix-plane structures are two other types of slow wave structures that relate to the present invention, certain aspects of such structures being disclosed by R. M. White, et al., in an article in IEEE Transactions on Electron devices, June 1964, pages 247-261. The ring-plane circuit is a series of axially spaced rings connected by radial support planes. The helix plane circuit is a helix supported by radial support planes. In their article, White et al., reported that measurements on the ring-plane structure indicated a very narrow bandwidth which makes such a circuit impractical for most applications. The article also taught away from the helix-plane type of structure on the grounds that it had essentially the same narrow bandwidth as the ring plane circuit. One aspect of the present invention is the discovery that the bandwidth of helix-plane structures is moderately high, much higher than the measurements reported by White, et al.
A major problem in all traveling wave tubes when operated as forward wave amplifiers is that they exhibit unwanted oscillation modes caused by backward waves, electromagnetic waves on the slow wave structure which flow in the direction opposite to that of the signal being amplified. These backward waves flow in a direction opposite to the direction of motion of the electron beam and cause unwanted oscillations and spurious signals. This characteristic is a direct result of the ability of slow wave structures such as described above to support numerous oscillation modes and can occur no mater how well matched are the input and output ends of the tube to the slow wave structure. Heretofore, numerous techniques have been used to prevent unwanted backward wave oscillations in traveling wave tubes. These techniques include introducing frequency selective losses tuned to the backward wave oscillation frequency and discontinuities in the slow wave structure which create two or more backward wave oscillation frequencies so that the circuit lacks enough length to start the unwanted oscillations.
These techniques work to some extent when applied to small size circuits such as the dielectrically supported helix, in which most of the offending modes are well removed in frequency from the desired forward propagation mode. However these techniques suffer a number of disadvantages among which are increased structure complexity and the introduction of undesired loss in the forward wave to be amplified. Furthermore, such techniques tend to lose their effectiveness in circuits having larger transverse dimensions, such as the ring-plane and helix-plane circuits, because a large number of backward wave modes can be supported in the general frequency range of the desired mode.